Metallurgical coke is produced from coal in a coke oven. The coking process involves the destructive distillation of a complex carbonaceous material. The compounds formed or driven off during the coking process have a wide range of boiling and melting points and solubilities. As a result, selective condensation or crystallization of the compounds with higher boiling points occurs, with consequent plugging of transmission lines, resulting in poor flow and all of the associated difficulties and dangers.
The gas transmission lines carrying coke oven gas can have up to 50% of their cross sectional area blocked by deposition due to (a) the dropping out of organic constituents, or (b) inorganic corrosion products formed by the hydrogen sulfide, cyanide, or thiocyanate contained in the gas acting on the metal piping. Where blast furnace gas is mixed with coke oven gas, iron oxide or other inorganic particles contribute to the reaction that results in corrosion products. Coke oven gas also is usually saturated with naphthalene and other readily sublimable hydrocarbons, such as anthracene and phenanthrene, and droplets of coal tar are almost always carried along throughout the gas system.
Deposits usually form at points of minimum velocity, or at sites of maximum surface to volume ratio, such as burner nozzles and orifice pins. The presence of these deposits limits the gas flow through the mains and can increase the pressure drop across the transmission distribution lines. As a result, proper distribution of the gas can be hindered. The hindrance is especially important for the underfiring system for heating coke ovens or for proper flame temperature control in boilers or reheating furnaces. A particular problem is plugging of the refractory-lined standpipes and goosenecks leading to the horizontal collecting main, which conducts the volatile products to the chemical recovery plant.
A method to prevent plugging of such lines during the refining of coke would be very desirable. One method that has met with success is the introduction of very powerful solvents into the system to dissolve and disperse the deposits. The solvent tends to liquify the deposits, with the liquid being removed from the line through a drip-leg. A most successful product that is used for this purpose contains the solvent N-methyl-2-pyrrolidinone (NMP). Unfortunately, NMP is an expensive solvent. A less expensive solvent system that would effectively prevent such plugging would be very desirable.